An electric double layer capacitor is an electric device comprising polarizable electrodes and an electrolyte. As the electrolyte, an electrolytic solution prepared by dissolving an electrolyte salt in water is generally used (JP-A-49-68254, etc.). The electric double layer capacitor has a function of storing electric charge in an electric double layer formed between the electrolytic solution and each electrode by polarization of the electrode. Such an electric double layer capacitor using an aqueous electrolyte is utilized as a small-sized power source for semiconductor memory backup, etc. and is under development as a large-sized power source for an automobile, etc.
Conventional electric double layer capacitors are roughly classified into two groups, i.e., those using an aqueous electrolyte, usually an aqueous sulfuric solution having a concentration of approximately 25-50% (JP-A-62-268119, JP-A-63-213915, JP-A-2-174210, etc.) and those using an organic solvent-based electrolyte (JP-A-49-68254, JP-A-7-86096, etc.). In general, the capacitors using an organic solvent-based electrolyte have a higher output voltage but are disadvantageous in that because of the low ionic conductivity of the electrolyte, the internal resistance is high, resulting in an insufficient output current. On the other hand, the capacitors using an aqueous electrolyte have a low output voltage but are advantageous in that because of the high ionic conductivity of the electrolyte, the internal resistance is low, resulting in a strong output current.
In addition, since the capacitors using an organic solvent-based electrolyte are flammable, the development of an electric double layer capacitor with a higher output using an aqueous electrolyte is desired from the viewpoint of safety.
However, for increasing the output of a large-sized power source obtained by connecting electric double layer capacitors using an aqueous electrolyte, as a plurality of cells in series or in parallel, the output current should be increased by reducing the internal resistance. The internal resistance is caused by the aqueous electrolyte, carbon as electrodes, collectors, etc. The internal resistance due to the aqueous electrolyte and carbon as electrodes can be reduced by thinning the cells of the capacitor, but the thinning decreases the capacity of each cell, so that the balance between capacity and voltage is lost.
A rubber film containing a conductive material is generally used as the collector (JP-A-2-174210, JP-A-4-240708, JP-A-5-299296, etc.). Such a conventional conductive rubber film, however, has a volume resistivity in the direction perpendicular to the surface of as high as approximately 8-500 .OMEGA.cm and hence makes it difficult to increase the output of the capacitor. Moreover, such a conductive rubber film is poor in acid resistance and hence is attacked by the acidic aqueous electrolyte when used for a long period of time, so that it is deteriorated in function or leaks the aqueous electrolyte in some cases.